In digital radio communications, an adaptive array antenna (hereinafter referred to as “AAA”) technology is used which controls directivities adaptively by adding weights to antenna outputs from a plurality of antenna elements. This AAA technology can suppress interference signals taking advantage that signals arrive from different directions and controlling directivities adaptively. Thus, the adaptive array antenna technology is an ideal method for eliminating interference signals on a same channel.
On the other hand, in digital radio communications, an interference canceller technology is used which estimates interference signals as well as desired signals based on maximum likelihood specifications. This interference canceller technology can eliminate interference signals from an identical direction, which cannot be eliminated by the AAA technology.
In recent years, an effort is underway to combine the AAA technology and interference canceller technology to make the most of features of both technologies. An example of this is a configuration combining the AAA and a Viterbi equalizer (MLSE: Maximum Likelihood Sequence Estimator) as a non-linear interference canceller (“Cascade Connection of Adaptive Array Least Square Combination and Non-Linear Interference Canceller and Characteristics” (Pukawa et al, 1996 Institute of Electronics, Information and Communication Engineers Communication Society Conference B-406)). In this configuration, the MLSE eliminates and detects interference signals in an identical direction which cannot be eliminated by the AAA, but the MLSE involves a problem that the amount of processing grows exponentially with respect to the number of interference signals to be eliminated. For this reason, the MLSE is generally used as an equalizer intended to eliminate interference between codes that occurs generally depending on the degree of delayed signals of the own channel.
However, a CDMA system has generally more multiple channels received at a same time and a same frequency than a TDMA (Time Division Multiple Access) system or FDMA (Frequency Division Multiple Access) system or other access systems. In the TDMA system, for example, only one channel signal exists in one slot and the own channel delayed signal is the only interference signal (delayed signals of previous slot signals are also added in high-speed transmission), while in the CDMA system, a plurality of channels are code-multiplexed in one slot.
The AAA in a cellular system is generally subject to restrictions on the number of antennas because of problems involving installation of antennas and power lines or from the standpoint of costs of its radio section and signal processing section and though the AAA focuses directivities, it actually has a beam width of 10 degrees or more. For this reason, it is difficult to spatially separate the AAA from other channels completely. Since, as described above, the CDMA system in particular has more other channel signals in the focused directivities than other access systems, the CDMA system includes more signals to be eliminated by an interference canceller. Thus, combining the AAA and MLSE in the CDMA system results in a problem that the amount of MLSE processing will increase exponentially and thereby increase the hardware scale.